How big are they? That depends on their albedo; measurements of TNO
albedos indicate that they may generally be slightly higher than the canonical
assumed value of 0.04. Pluto has a comparatively high albedo (
0.50).
Below is the diameter (in km) as a function of albedo from the table at
http://cfa-www.harvard.edu/iau/lists/Sizes.html:

IAUC 8577 also reports the discovery by Brown et al. that 2003 EL61 is a
binary, with a period of days, a semimajor axis
of km, and a total system mass
about a third of the mass of the Pluto-Charon system.
Details are given in the paper listed in this issue of the Newsletter.

The stellar occultation by Charon was the target of an observing campaign
reported in IAUC 8570. The results reported by L.A. Young et al. give a
lower limit on Charon's diameter of 11794 km, and no detection of an
atmosphere.

Several days after the above announcement of cometary activity in Centaur
2004 PY42, IAUC 8552 presented an editorial from the Minor Planet Center about
the difficulty in having consistent nomenclature rules for cometary Centaurs
and TNOs, not only for the numbering scheme, but also
for the names (mythical creatures vs. the discoverers; typically the two
do not overlap). A hybrid solution will be followed: rather than use the
cometary rules for numbering
(observations at two or more perihelion passages, which would be unwieldy
for Centaurs), the MPC will continue to use the criteria used for Centaurs
and TNOs, i.e., a well-known orbit (MPC orbit uncertainty parameter < 3)
and observations during four or more oppositions (with at least one recent).
The object's name will be that of the discoverer(s), following the convention for
comets. Also, note that such objects will no longer appear in the MPC's
Distant Object databases, but will be moved to the comet databases.

N-body simulations are used to examine the consequences of
Neptune's outward migration into the Kuiper Belt, with the
simulated endstates being compared rigorously and quantitatively
to the observations.
These simulations confirm the findings of Chiang et al. (2003),
who showed that Neptune's migration into a previously stirred-up
Kuiper Belt can account for the Kuiper Belt Objects
(KBOs) known to librate at Neptune's 5:2 resonance. We also
find that capture is possible at many other weak,
high-order mean motion resonances, such as the
11:6, 13:7, 13:6, 9:4, 7:3, 12:5, 8:3, 3:1, 7:2, and the 4:1.
The more distant of these resonances, such as the 9:4, 7:3, 5:2,
and the 3:1, can also capture particles in stable, eccentric orbits
beyond 50 AU, in the region of phase space conventionally known as the
Scattered Disk. Indeed, 90% of the simulated particles
that persist over the age of the Solar System in the so-called
Scattered Disk zone never had a close encounter with Neptune, but
instead were promoted into these eccentric orbits by Neptune's
resonances during the migration epoch. This indicates that the observed
Scattered Disk might not be so scattered. This model also produced
only a handful of Centaurs, all of which originated at
Neptune's mean motion resonances in the Kuiper Belt. However
a noteworthy deficiency of the migration model considered here
is that it does not account for the observed abundance of Main Belt KBOs
having inclinations higher than 15 degrees.

In order to rigorously compare the model endstate with the observed
Kuiper Belt in a manner that accounts for telescopic selection effects,
Monte Carlo methods are used to assign sizes and magnitudes to the
simulated particles that survive over the age of the Solar System.
If the model considered here
is indeed representative of the outer Solar System's early
history, then the following conclusions are obtained:
(i) the observed 3:2 and 2:1 resonant populations
are both depleted by a factor of 20 relative to model expectations;
this depletion is likely due to unmodeled effects, possibly perturbations
by other large planetesimals,
(ii) the size distribution of those KBOs inhabiting the 3:2
resonance is significantly shallower than the Main Belt's size distribution,
(iii) the total number of KBOs having radii R>50 km and orbiting
interior to Neptune's 2:1 resonance is
; these bodies
have a total mass of
Earth-masses assuming they have a material density rho and an albedo p.
We also report estimates of the abundances and masses of the Belt's various
subpopulations (e.g., the resonant KBOs, the Main Belt, and the
so-called Scattered Disk), and also provide upper limits on the abundance of
Centaurs and Neptune's Trojans, as well as upper limits on the sizes and abundances
of hypothetical KBOs that might inhabit the a>50 AU zone.

A numerical study of an ensemble of orbits based on
observed objects in the near-Neptune high-eccentricity (NNHE) region, with
perihelion distances q in the range 28<q<35.5 AU and semimajor axes
a in the range 60< a < 1000 AU, is used to predict the orbital
distribution of Centaurs (5<q<28 AU) for comparison with observations
after correcting for discovery biases. The majority of Centaurs produced
in this way have
AU. However, the intrinsic number of
observed Centaurs is dominated by longer period objects, the number with
a > 60 AU being roughly an order of magnitude greater than that for
a < 60 AU, and therefore inconsistent with a source in the NNHE
region, which is broadly similar to the so-called `Scattered Disc'. The
observed distribution of Centaurs with
AU is also
inconsistent with this source, although it is conceivable that in this
region the discrepancies might be explained by factors such as outgassing,
splitting or varying albedo not included in our model. Thus, although
Centaurs can be produced from the NNHE region, their numbers and orbital
distributions are inconsistent with this region being the dominant source
for all Centaurs. We conclude that there must be another source flux,
especially for the longer period, more populous group, and suggest that
the most likely source for these objects is the Oort cloud. Thus, there
are two separate, but overlapping dynamical classes of Centaurs, one
originating from the Oort cloud and the other from the NNHE region. The
two source regions produce roughly similar contributions to Centaurs with
AU and to the observed Jupiter family of comets.

1 School of Mathematics, University of Bristol,
University Walk, Bristol BS8 1TW, UK

In this paper we use recently developed phase-space transport
theory coupled with a so-called classical spectral theorem to
develop a dynamically exact and computationally efficient
procedure for studying escape from a planetary neighbourhood. The
`planetary neighbourhood' is a bounded region of phase space
where entrance and escape are only possible by entering or
exiting narrow `bottlenecks' created by the influence of a saddle
point. The method therefore immediately applies to, for example,
the circular restricted three-body problem and Hill's lunar
problem (which we use to illustrate the results), but it also
applies to more complex, and higher-dimensional, systems
possessing the relevant phase-space structure. It is shown how
one can efficiently compute the mean passage time through the
planetary neighbourhood, the phase-space flux in, and out, of the
planetary neighbourhood, the phase-space volume of initial
conditions corresponding to trajectories that escape from the
planetary neighbourhood, and the fraction of initial conditions
in the planetary neighbourhood corresponding to bound
trajectories. These quantities are computed for Hill's
problem. We study the dependence of the proportions of these
quantities on energy and dimensionality (two-dimensional planar
and three-dimensional spatial Hill's problem). The methods and
quantities presented are of central interest for many celestial
and stellar dynamical applications such as, for example, the
capture and escape of moons near giant planets, the formation of
binaries in the Kuiper belt and the escape of stars from star
clusters orbiting about a galaxy.

Published in:
Monthly Notices of the Royal Astronomical Society, 361, 763

The discovery that many trans-Neptunian objects exist in pairs,
or binaries, is proving invaluable for shedding light on the
formation, evolution and structure of the outer Solar
system. Based on recent systematic searches it has been estimated
that up to 10 per cent of Kuiper-belt objects might be
binaries. However, all examples discovered to date are unusual,
as compared with near-Earth and main-belt asteroid binaries, for
their mass ratios of the order of unity and their large,
eccentric orbits. In this article we propose a common dynamical
origin for these compositional and orbital properties based on
four-body simulations in the Hill approximation. Our calculations
suggest that binaries are produced through the following chain of
events. Initially, long-lived quasi-bound binaries form by two
bodies getting entangled in thin layers of dynamical chaos
produced by solar tides within the Hill sphere. Next, energy
transfer through gravitational scattering with a low-mass
intruder nudges the binary into a nearby non-chaotic, stable zone
of phase space. Finally, the binary hardens (loses energy)
through a series of relatively gentle gravitational scattering
encounters with further intruders. This produces binary orbits
that are well fitted by Kepler ellipses. Dynamically, the overall
process is strongly favoured if the original quasi-bound binary
contains comparable masses. We propose a simplified model of
chaotic scattering to explain these results. Our findings suggest
that the observed preference for roughly equal-mass ratio
binaries is probably a real effect; that is, it is not primarily
due to an observational bias for widely separated, comparably
bright objects. Nevertheless, we predict that a sizeable
population of very unequal-mass Kuiper-belt binaries is probably
awaiting discovery.

Published in:
Monthly Notices of the Royal Astronomical Society, 360, 401

Sedna is, so far, the largest and most distant trans-neptunian object.
It was observed at visible and near-infrared wavelengths using simultaneously two 8.2 m
telescopes at the Very Large Telescope of the European Southern Observatory.
The spectrum of Sedna suggests the presence on its surface of different ices (total
abundance > 50%). Its surface composition is different from that determined for
other trans-neptunian objects, and apparently resembles that of Triton, particularly
in terms of the possible presence of nitrogen and methane ices.

We present precise, , r-band relative photometry of the
unusual solar system object (90377) Sedna. Our data consist of 143
data points taken over eight nights in October 2004 and
January 2005. The RMS variability over the longest contiguous stretch
of five nights of data spanning nine days is only . This
subset of data alone constrains the amplitude of any long-period
variations with period P to be
. Over the
course of any given -hour segment, the data exhibit
significant linear trends not seen in a comparison star of similar
magnitude, and in a few cases these segments show clear evidence for
curvature at the level of a few millimagnitudes per hour2. These
properties imply that the rotation period of Sedna is
,
cannot be < 5 hours, and cannot be > 10 days, unless the
intrinsic light curve has significant and comparable power on multiple
timescales, which is unlikely. A sinusoidal fit yields a period of
hours and semi-amplitude of
.
There are additional acceptable fits with flanking periods separated by
minutes, as well as another class of fits with hours, although these later fits appear less viable based on
visual inspection.
Our results indicate that the period of Sedna is likely consistent
with typical rotation periods of solar system objects, thus obviating
the need for a massive companion to slow its rotation.

We present optical photometry of the Centaur 5145 Pholus during
2003 May and 2004 April using the facility CCD camera on the
1.8-m Vatican Advanced Technology Telescope on Mt. Graham,
Arizona. We derive a double-peaked lightcurve and a rotation
period of
h for Pholus, consistent with periods of
and
h by Buie and Bus (1992, Icarus
100, 288 294) and Farnham (2001, Icarus 152, 238 245). We find a
lightcurve peak-to-peak amplitude of 0.60 mag, significantly
larger than peak-to-peak amplitude determinations of 0.15 and
0.39 mag by Buie and Bus and Farnham. We use the three observed
amplitudes and an amplitude-aspect model to derive four possible
rotational pole positions as well as axial ratios of a/b=1.9 and
c/b=0.9. If we assume an albedo of 0.04, we find Pholus has
dimensions of
310 x 160 x 150 km. If we assume Pholus is a
strengthless rubble-pile and its non-spherical shape is due to
rotational distortion, our axial ratios and period measurements
indicate Pholus has a density of 0.5 g cm-3, suggestive of an
ice-rich, porous interior. By combining B-band and R-band
lightcurves, we find
and any B-R color variation
over the surface of Pholus must be smaller than 0.06 mag (i.e.,
much smaller than the 1.0<B-R<2.0 range seen among the Centaur
and Kuiper belt object populations). By combining our V-R
measurements with values in the literature, we find no evidence
for any color variegation between the northern and southern
hemispheres of Pholus. Observations of the Kuiper belt object
2004 DW (90482) over a time interval of seven hours show no color
variation Our observations add to the growing body of evidence
that individual Centaurs and KBOs exhibit homogeneous surface
colors and hence gray impact craters on radiation reddened crusts
are probably not responsible for the surprising range of colors
seen among the Centaur and Kuiper belt object populations.

We present photometric observations of Centaur (60558) 2000 EC98
and trans-neptunian object (55637) 2002 UX25 at different
phase angles and with different filters (mainly R but also V and B
for some data). Results for 2000 EC98 are: (i) a rotation period of
hours if a double-peaked lightcurve is assumed,
(ii) a lightcurve amplitude of for the R band,
(iii) a phase curve with
and
(R filter)
and
and
(V filter) or a slope
of mag deg-1 (R filter) and (V filter),
(iv) the color indices
and
(for
-0.5 deg) and (for
-1.5 deg). The rotation period
is amongst the longest ever measured for Centaurs and TNOs.
We also show that our photometry was not contaminated by any cometary
activity down to magnitude arcsec-2.

For 2002 UX25 the results are:
(i) a rotation period of
hours or
hours (if a double-peaked lightcurve is assumed)
(ii) a lightcurve amplitude of for the R band (and the
16.782 hours period),
(iii) a phase curve with
and
or a slope of mag deg-1 (R filter),
(iv) the color indices
and
.
The phase curve reveals also a possible very narrow and bright opposition
surge. Because such a narrow surge appears only for one point it needs
to be confirmed.

We detected thermal emission from the Kuiper Belt object
2002 AW197 in 2003 December and again in 2004 April using the
Multiband Imaging Photometer on the Spitzer Space Telescope. In
combination with the absolute visual magnitude, the thermal
measurements indicate a geometric albedo of and a
diameter of km. The albedo of 2002 AW197 is
significantly higher than the 0.04 value typically assumed for
trans-Neptunian objects, and consequently the object is smaller
than previously thought based on that assumption. Our thermal
measurements at two wavelengths (24 and 70 m) allow us to
constrain the surface temperature and thereby place constraints
on the thermal inertia. We find that the standard thermal
model (STM) is inconsistent with the 24/70 m color unless we
set the beaming parameter , indicating that the object
has a significant thermal inertia and, therefore, that the STM is
inappropriate. The other end-member thermal inertia model is the
fast-rotator, or isothermal-latitude, model (ILM). The data are
well represented by an ILM with the pole of rotation inclined to
the Sun by deg. The high albedo is consistent with a
surface containing significant amounts of weakly absorbing
materials, with ices and/or fine-grained silicates as likely
candidates.

Transneptunian objects (TNOs) orbit beyond Neptune and do offer
important clues about the formation of our solar system. Although
observations have been increasing the number of discovered TNOs
and improving their orbital elements, very little is known about
elementary physical properties such as sizes, albedos and
compositions. Due to TNOs large distances (>40 AU) and
observational limitations, reliable physical information can be
obtained only from brighter objects (supposedly larger
bodies). According to size and albedo measurements available, it
is evident the traditionally assumed albedo p=0.04 cannot hold
for all TNOs, especially those with approximately absolute
magnitudes . That is, the largest TNOs possess higher
albedos (generally >0.04) that strongly appear to increase as a
function of size. Using a compilation of published data, we
derived empirical relations which can provide estimations of
diameters and albedos as a function of absolute
magnitude. Calculations result in more accurate size/albedo
estimations for TNOs with than just assuming
p=0.04. Nevertheless, considering low statistics, the value
p=0.04 sounds still convenient for H>5.5 non-binary TNOs as a
group. We also discuss about physical processes (e.g.,
collisions, intrinsic activity and the presence of tenuous
atmospheres) responsible for the increase of albedo among large
bodies. Currently all big TNOs (>700 km) would be capable to
sustain thin atmospheres or icy frosts composed of CH4, CO or N2
even for body bulk densities as low as 0.5 g cm-3. A
size-dependent albedo has important consequences for the TNOs
size distribution, cumulative luminosity function and total mass
estimations. According to our analysis, the latter can be reduced
up to 50% if higher albedos are common among large bodies.
Lastly, by analyzing orbital properties of classical
TNOs (42 AU<a<48 AU), we confirm that cold and hot classical TNOs
have different concentration of large bodies. For both
populations, distinct absolute magnitude distributions are
maximized for an inclination threshold equal 4.5 degrees at
>99.63% confidence level. Furthermore, more massive classical
bodies are anomalously present at a<43.5 AU, a result
statistically significant and apparently not caused by
observational biases. This feature would provide a new constraint
for transneptunian belt formation models.

Over the last 13 years, the outer solar system has been found to
be densely populated by many bodies called Kuiper Belt or
Trans-Neptunian Objects. These discoveries have opened up a new
frontier in solar system astronomy. The scientific community has
developed scenarios for the understanding of the Kuiper
Belt. This field of research continues to evolve rapidly with the
promise of an exciting future.

For these reasons, the time has
come to begin work on a Kuiper Belt book to be published in the
Space Science Series of the University of Arizona Press. A
Scientific Organizing Committee (SOC) has been formed and has
made preliminary plans for the organization and content of this
book.

The Distant EKOs Newsletter is dedicated to provide researchers with
easy and rapid access to current work regarding the Kuiper belt (observational
and theoretical studies), directly related objects (e.g., Pluto, Centaurs), and
other areas of study when explicitly applied to the Kuiper belt.

Recent and back
issues of the Newsletter are archived there in various formats. The web
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